Looking at entries in the Scott US Specialized catalog, youíll notice price changes based on spacing varieties in pairs or blocks of some early US stamps. For example, Scott #384 mentions 2 and 3 mm spacing varieties. Why is this?
When flat plate stamps are printed, the sheets of paper used are damp. The high moisture content allows the paper to more easily absorb the ink. However, as the paper dries, the shrinkage is uneven. The outer edges of the sheet of paper shrink more than the internal areas of the sheet. After the sheets of stamps are dry, they are perforated. The metal perforating wheels are evenly aligned. However, the perforations on the outer edges of the sheets of stamps would cut into the stamp design because of the uneven shrinkage. Stamps in the center of the sheet may be beautifully perforated with nice even margins all around while outer rows of stamps are very poor or unacceptable by quality standards.
To combat this problem, the Bureau of Engraving and Printing started using different plate layouts in 1909. The outer seven vertical rows of stamps had 3 mm. spacing between the stamps. All other vertical rows of stamp had 2 mm. spacing. Remember, most sheets of stamps were printed 400 stamps at a time. There were 4 panes of 100 stamps each on a sheet.
On a pane of 100 stamps, the 7 leftmost or 7 rightmost vertical rows of stamps (depending on whether the pane came from the left or right side of the full sheet) will have the 3 mm. spacing and the other 3 vertical rows of stamps will have the 2 mm. spacing. Therefore, pairs and blocks with 3 mm. spacing are twice as common as pairs and blocks with 2 mm. spacing. The price differences arenít huge between the two spacing varieties. But it does recognize that the 2 mm. spacing is not as common.
Now, the BEP did experiment with other spacing variations in order to get this as perfect as possible and reduce the number of poor quality stamps. Donít be surprised if you find pairs and blocks of stamps where the spacing isnít exactly matching the 2 or 3 mm. measurement. And remember, as the paper shrinks during drying, it can skew the measurements. Variations of 1.4 or 1.2 mm. here or there are typical.
As the BEP moved to printing stamps on rotary printing presses (using large, continuous rolls of paper) and especially the move to dry printed stamps, the problems with uneven paper shrinkage were eliminated.
Finding these pairs and blocks with various spacing differences can be kind of challenging. Or if you really want a challenge, find larger strips or blocks of stamps that are 3 stamps or more wide and one or more vertical rows shows a 3 mm spacing and the other rows show the 2 mm. spacing.